Luminance of an LED depends on a current flowing through it, so an LED drive circuit is required to convert mains supply into constant current output or constant voltage output so as to drive the LED. Under occasions of phase-control dimming, an LED dimmer circuit may include a phase-control light dimmer and an LED drive circuit.
FIG. 1 illustrates a schematic diagram of a prior art phase-control light dimmer. As shown in FIG. 1, holding current is required for conduction of triac (DIAC, TRIC). Therefore, in order to realize LED dimming, a fixed load (also known as a dead load or a bleeder resistor) is added in many types of LED driver circuits to hold a minimum breakover current during conduction, thus enabling a phase angle signal for dimming control to be detected accurately. For example, FIG. 2 illustrates an LED dimmer circuit employing the phase-control light dimmer of FIG. 1, an LM3445 scheme adopted by U.S. National Semiconductor Corporation. However, additional fixed loads, such as dead load 21 in FIG. 2, may cause relatively great loss, which may reduce efficiency of an LED drive circuit.
In some occasions, a PFC (power factor correction) scheme is adopted for a prior LED drive circuit. During conduction of the phase-control light dimmer, input current and input voltage of the drive circuit are same in phase and proportional in amplitude. The LED drive circuit may use an output load as a load of a light dimmer, thus leaving out a dead load. By sampling a conduction angle of input voltage of the drive circuit, the LED drive circuit filters the conduction angle and converts it into a DC signal whose amplitude is in direct proportion to the conduction angle of dimming signal output by the phase-control light dimmer. The DC signal is used as a reference for comparing with an output current, so as to output an error signal and to control magnitude of the output current. However, this dimming mode has problems of startup flickering and failure of wide range dimming. Specifically speaking, if the phase-control light dimmer is started up when the conduction angle is relatively small, a certain delay time exists when the conduction angle of the input voltage is filtered and converted into the DC signal, and the DC signal within the delay time fails to reflect change of the phase angle for the input voltage in a rapid and accurate way, thus leading to higher current reference and heavier output current within the delay time, which may cause flickering. Moreover, there may be also problems of complex reference sampling circuits and excessive elements.